The color temperature of light emitted by a hollow prismatic light guide luminaire is varied by yellow hue filtering flare light near the light guide""s light input end to counteract aesthetically undesirable far-end reddening gradient of the emitted light.
FIGS. 1A-1C depict a prior art hollow prism light guide luminaire 10 generally representative of those described in U.S. Pat. Nos. 4,615,579; 4,750,798; and, 4,787,708 (Whitehead). As explained in U.S. Pat. No. 5,339,382 (Whitehead) light rays emitted by light source 12 are guided along and confined within light guide 14 by means of total internal reflection. Light guide 14 is housed within an opaque cover 16 having a reflective inner surface 18 and a light emitting aperture 20. One or more white-colored diffuse light extractors 22 are provided within guide 14. Light rays guided along guide 14 occasionally strike extractor 22, causing a random change in the direction of such rays; usually into a direction which guide 14 is unable to confine by total internal reflection, thus allowing such rays to escape from guide 14. For example, a light ray originating at point 24 is reflected by extractor 22 and strikes guide 14 at an angle which results in further reflection of the ray such that it escapes through the wall of guide 14 and is emitted through aperture 20 in a direction 26. Similarly, a light ray originating at point 28 strikes extractor 22, escapes through guide 14 and is emitted through aperture 20 in direction 30.
Light rays emitted through aperture 20 can be used for interior space illumination. In such case, luminaire 10 is preferably configured to emit substantially uniformly bright light through all points along aperture 20 as is for example explained in U.S. Pat. No. 4,850,665 (Whitehead). But, because the refractive index and light transmissivity characteristics of the dielectric material used to form light guide 14 vary as a function of wavelength, an aesthetically undesirable color gradient is observed along luminaire 10. More particularly, light guide 14 absorbs some blue light rays, so as distance from light source 12 increases, progressively fewer blue light rays are guided along light guide 14. Consequently, light emitted through aperture 20 at distances farther from light source 12 is perceived as more xe2x80x9credxe2x80x9d than light emitted through aperture 20 at distances closer to light source 12, even if the emitted light is uniformly bright at all points on aperture 20. This color temperature drop or far-end xe2x80x9creddeningxe2x80x9d of light guide 14 as a function of distance from light source 12 is on the order of 600 degrees Kelvin relative to a nominal correlated color temperature for a typical light guide having a length LG greater than 20 times the light guide""s diameter DG. FIG. 2A graphically illustrates the color temperature drop along light guide 14 and also shows that luminaire 10""s luminance characteristic is reduced (typically by as much as 30%) as a function of distance along light guide 14 due to light absorption losses.
Preferably, no visually perceptible color gradient is observable along luminaire 10. A typical prior art technique for reducing the observable color gradient is to vary the color of extractor 22 as a function of distance from light source 12 along light guide 14. Typically, extractor 22 is located at the end of light guide 14 farthest from light source 12 and has a light transmissivity characteristic which varies as a function of distance from light source 12 along light guide 14, to achieve the desired uniformly bright light emission through all points along aperture 20. If extractor 22 has a blue color and varies in width as a function of distance from light source 12 along light guide 14 (less blue extractor material at the end of extractor 22 closest to light source 12 and progressively more blue extractor material toward the end of extractor 22 farthest from light source 12) reddening of light guide 14 is offset since extractor 22 preferentially passes blue light while absorbing red light as a function of distance from light source 12 along light guide 14. As FIG. 2B graphically illustrates, the red absorption required to effectively offset far-end reddening of light guide 14 reduces luminaire 10""s luminance characteristic by as much as 40% by absorbing a significant fraction (as much as 10%) of the red light rays guided along light guide 14. Such absorption losses are unacceptable in many lighting situations since they may require a more expensive light source having greater light output capability to achieve a desired minimum output luminance. Moreover, because extractor 22""s shape typically varies along light guide 14 as a function of distance from light source 12 (see FIGS. 1A and 1C) luminaire 10""s perceived color depends on the observer""s viewing angle relative to luminaire 10, which is aesthetically undesirable.
This invention offsets far-end reddening in a hollow prismatic light guide luminaire without significantly reducing the luminaire""s luminance characteristic.
The invention facilitates color temperature variation of light emitted by a luminaire having a hollow prismatic light guide formed of a material which absorbs blue light rays such that more red light rays are guided along the light guide at distances father from a light input end of the light guide than at distances closer to the light input end. The desired color temperature variation is achieved by color filtering flare light rays which escape through the light guide at points close to the light input end and reflecting the color filtered light rays back into the light guide. The filter color lies within a CIE-1931 chromaticity diagram {4800xc2x0 Kelvin; 570 nm; 600 nm} color gamut triangle.
The amount of color filtering is advantageously varied as a function of distance along the light guide and in inverse proportion to the absorption of blue light rays by the light guide material. This can be achieved by varying the filter""s intensity (color saturation) as a function of distance along the light guide, or by varying the filter""s width as a function of distance along the light guide, or by varying both the filter""s color intensity and width as a function of distance along the light guide.